1. Field of the Invention
This invention relates to a method for producing single wall carbon nanotubes, also known as linear fullerenes, employing unsupported metal containing catalysts, for decomposition of a C1 to C6 carbon feedstock such as carbon monoxide.
2. Description of the Related Art
Multi-Walled Carbon Nanotubes
Multi-walled carbon nanotubes, or fibrils, are well-known. Typically, carbon fibrils have a core region comprising a series of graphitic layers of carbon.
Since the 1970""s, carbon nanotubes and fibrils have been identified as materials of interest for a variety of applications. Submicron graphitic fibrils belong to a class of materials sometimes called vapor grown carbon fibers. Carbon fibrils are vermicular carbon deposits having diameters less than approximately 1.0 xcexc. They exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Such vermicular carbon deposits have been observed almost since the advent of electron microscopy. A good early survey and reference is found in Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83, and in Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993).
Carbon fibrils were seen to originate from a metal catalyst particle which, in the presence of a hydrocarbon containing gas, became supersaturated in carbon. A cylindrical ordered graphitic core is extruded which immediately became coated with an outer layer of pyrolytically deposited graphite. These fibrils with a pyrolytic overcoat typically have diameters in excess of 0.1 xcexc. (Oberlin, A. and Endo, M., J. Crystal Growth, 32:335-349 (1976)).
Tibbetts has described the formation of straight carbon fibers through pyrolysis of natural gas at temperatures of 950xc2x0-1075xc2x0 C., Appl. Phys. Lett. 42(8):666(18 983). The fibers are if reported to grow in two stages where the fibers first lengthen catalytically and then thicken by pyrolytic deposition of carbon. Tibbetts reports that these stages are xe2x80x9coverlappingxe2x80x9d, and is unable to grow filaments free of pyrolytically deposited carbon. In addition, Tibbett""s approach is commercially impracticable for at least two reasons. First, initiation of fiber growth occurs only after slow carbonization of the steel tube (typically about ten hours), leading to a low overall rate of fiber production. Second, the reaction tube is consumed in the fiber forming process, making commercial scale-up difficult and expensive.
In 1983, Tennent, U.S. Pat. No. 4,663,230 succeeded in growing cylindrical ordered graphite cores, uncontaminated with pyrolytic carbon, resulting in smaller diameter fibrils, typically 35 to 700 xc3x85 (0.0035 to 0.070 xcexc), and an ordered xe2x80x9cas grownxe2x80x9d graphitic surface. Tennent ""230 describes carbon fibrils free of a continuous thermal carbon overcoat and having multiple graphitic outer layers that are substantially parallel to the fibril axis. They may be characterized as having their c-axes, (the axes which are perpendicular to the tangents of the curved layers of graphite) substantially perpendicular to their cylindrical axes, and having diameters no greater than 0.1 xcexc and length to diameter ratios of at least 5.
Tennent, et al., U.S. Pat. No. 5,171,560 describes carbon fibrils free of thermal overcoat and having graphitic layers substantially parallel to the fibril axes such that the projection of said layers on said fibril axes extends for a distance of at least two fibril diameters. Typically, such fibrils are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise cylindrical graphitic sheets whose c-axes are substantially perpendicular to their cylindrical axis. They are substantially free of pyrolytically deposited carbon, have a diameter less than 0.1 xcexc and a length to diameter ratio of greater than 5.
Moy et al., U.S. Ser. No. 07/887,307 filed May 22, 1992, describes fibrils prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) including morphologies resembling bird nests (xe2x80x9cBNxe2x80x9d), combed yarn (xe2x80x9cCYxe2x80x9d) or xe2x80x9copen netxe2x80x9d (xe2x80x9cONxe2x80x9d) structures.
Multi-walled carbon nanotubes of a morphology similar to the catalytic grown fibrils described above have been grown in a high temperature carbon arc (Iijima, Nature 354:56 1991). (Iijima also describes in a later publication arc-grown single-walled nanotubes having only a single layer of carbon arranged in the form of linear Fullerene.) It is now generally accepted (Weaver, Science 265: 1994) that these arc-grown nanofibers have the same morphology as the earlier catalytically grown fibrils of Tennet.
As mentioned above, the Iijima method partially results in single-walled nanotubes, i.e., nanotubes having only a single layer of carbon arranged in the form of a linear Fullerene.
U.S. Pat. No. 5,424,054 to Bethune et al. describes a process for producing single-walled carbon nanotubes by contacting carbon vapor with cobalt catalyst. The carbon vapor is produced by electric arc heating of solid carbon, which can be amorphous carbon, graphite, activated or decolorizing carbon or mixtures thereof. Other techniques of carbon heating are discussed, for instance laser heating, electron beam heating and RF induction heating.
Smalley (Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smally, R. E., Chem. Phys. Lett. 243: 1-12 (1995)) describes a method of producing single-walled carbon nanotubes wherein graphite rods and a transition metal are simultaneously vaporized by a high-temperature laser.
Smalley (Thess, A., Lee, R., Nikolaev, P. Dai, H, Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G. Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonàrek, D., Fischer, J. E., and Smalley, R. E., Science 273: 483-487 (1996)) also describes a process for production of single-waved carbon nanotubes in which a graphite rod containing a small amount of transition metal is laser vaporized in an oven at about 1200xc2x0 C. Single-wall nanotubes were reported to be produced in yields of more than 70%.
Each of the techniques described above employs solid carbon as the carbon feedstock. These techniques are inherently disadvantages. Specifically, solid carbon vaporization via electric arc or laser apparatus is costly and difficult to operate on the commercial or industrial scale.
Supported metal catalysts for formation of SWNT are also known. Smalley (Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 260: 471-475 (1996)) describes supported Co, Ni and Mo catalysts for growth of both multi-walled nanotubes and single-walled nanotubes from CO, and a proposed mechanism for their formation.
However, supported metal catalysts are inherently disadvantageous, as the support is necessarily incorporated into the single-walled carbon nanotube formed therefrom. Single-walled nanotubes contaminated with the support material are obviously less desirable compared to single-walled nanotubes not having such contamination.
It is thus an object of the present invention to provide a method of producing single-walled carbon nanotubes which employs a gaseous carbon feedstock.
It is an object of this invention to provide a method of producing single-walled carbon nanotubes which employs a gas phase, metal containing compound which forms a metal containing catalyst.
It is also an object of the invention to provide a method of producing single-walled carbon nanotubes which employs an unsupported catalyst.
It is a further object of this invention to provide a method of producing single-walled carbon nanotubes which employs a gaseous carbon feedstock and an unsupported gas phase metal containing compound which forms a metal containing catalyst.
The invention relates to a gas phase reaction in which a gas phase metal containing compound is introduced into a reaction mixture also containing a gaseous carbon source. The carbon source is typically a C1 through C6 compound having as hetero atoms H, O, N, S or Cl, optionally mixed with hydrogen. Carbon monoxide or carbon monoxide and hydrogen is a preferred carbon feedstock.
Increased reaction zone temperatures of approximately 400xc2x0 C. to 1300xc2x0 C. and pressures of between xe2x88x920 and xe2x88x92100 p.s.i.g., are believed to cause decomposition of the gas phase metal containing compound to a metal containing catalyst. Decomposition may be to the atomic metal or to a partially decomposed intermediate species. The metal containing catalysts (1) catalyze CO decomposition and (2) catalyze SWNT formation. Thus, the invention also relates to forming SWNT via catalytic decomposition of a carbon compound.
The invention may in some embodiments employ an aerosol technique in which aerosols of metal containing catalysts are introduced into the reaction mixture. An advantage of an aerosol method for producing SWNT is that it will be possible to produce catalyst particles of uniform size and scale such a method for efficient and continuous commercial or industrial production. The previously discussed electric arc discharge and laser deposition methods cannot economically be scaled up for such commercial or industrial production.
Examples of metal containing compounds useful in the invention include metal carbonyls, metal acetyl acetonates, and other materials which under decomposition conditions can be introduced as a vapor which decomposes to form an unsupported metal catalyst.
Catalytically active metals include Fe, Co, Mn, Ni and Mo. Molybdenum carbonyls and Iron carbonyls are the preferred metal containing compounds which can be decomposed under reaction conditions to form vapor phase catalyst. Solid forms of these metal carbonyls may be delivered to a pretreatment zone where they are vaporized, thereby becoming the vapor phase precursor of the catalyst.